This relates generally to imaging systems and, more particularly, to signal processing circuitry that utilizes pinned photodiode devices for delivering charge to a circuit node.
Modern electronic devices such as cellular telephones, cameras, and computers often include camera modules having digital image sensors. An image sensor (sometimes referred to as an imager) is formed from a two-dimensional array of image sensing pixels. Each pixel receives incident photons (light) and converts the photons into electrical signals.
Capturing images using an image sensor involves using reading out pixel signals from a subset of pixels from the two-dimensional image sensing pixel arrays (sometimes referred to as a “readout operation” of an image sensor). Pixel signals may be routed or otherwise provided to signal processing circuitry during the readout operation. A readout operation may be said to conclude when the signal processing circuitry that receives the image pixel signals converts the image pixel signals to digital image data. Prior to the read out of pixel signals from a subset of the pixels in an array, the reset levels from the subset of the pixels in the array are also read out and converted to digital reset level data by the signal processing circuitry on the image sensor.
Converting pixel reset levels and pixel signals from analog signals to digital data is accomplished by analog-to-digital converter (ADC) circuitry. Conventional ADC circuits sometimes utilize poly-insulator-poly or metal-insulator-metal capacitors having large substrate area requirements, density requirements, linearity requirements, and extra silicon processing steps to form them. Capacitors may be used in switch capacitor circuits that provide reference charges to the comparator circuitry in the ADC circuitry. Comparator circuitry in the traditional ADC circuitry itself often requires capacitors. The capacitors in ADC circuitry are often used to transfer large amount of charges between nodes, resulting in excessive power consumption and dissipation in the signal processing circuitry. Moreover, capacitors that are formed in signal processing circuitry are not customizable as far as specialized silicon processing needed to meet capacitor device performance specifications, thereby limiting the applications and configurability of an image sensor that relies on capacitors to provide references charges for an ADC circuit.
An image sensor that lacks silicon process customization or configurability for specialized analog circuit components in its image processing circuitry cannot be optimized for particular applications as readily, if at all, when compared to sensors having configurable processing circuitry. Furthermore, reliance on capacitors to transfer large charge packets between nodes often results in excess power consumption and dissipation in the signal processing circuitry, further limiting the applicability of the capacitor-based signal processing circuitry to systems with larger and more costly power sources and heat dissipation capabilities that are suited to the power requirements of the capacitor-based circuitry. Capacitors used in ADC circuitry are also used to charge mixing, which occurs when capacitors are connected together or coupled to a common node and settle to a common voltage. A capacitor in signal processing circuitry with a charge level transfers charges to a second capacitor at a lower charge level when an electrical path is formed between the two capacitors, resulting in a mixing of capacitor signals when the two capacitors are not electrically isolated from one another like with an amplifier in a switched capacitor circuit topology.
It would therefore be desirable to provide improved signal processing circuitry without reliance on conventional high performance capacitors that dissipate power to support charge mixing or dissipate power to support switched capacitor circuit topologies.